Producing droplets of predictable size within a narrow droplet size distribution has been the admirable goal of many prior art attempts. Heat and mass transfer characteristics, as well as other process parameters, change significantly for droplets within the range of diameters typically produced by many prior art devices. Process calculations for modeling such processes with wide droplet size distribution must be subdivided into size groupings and require sophisticated computer-based solutions. Actual operation of processes with wide droplet size distribution generally produces results which are less stable and less predictable than those in which droplet size is effectively narrowed.
Capillary wave atomization is done with two general types of devices. U.S. Pat. No. 5,687,905 shows one of the types, the nozzle type, where liquid runs through a conduit inside a metal cone to a tip. The nozzle consists of a transducer located at a node in the nozzle axis and rigidly connected with two separated masses, where each mass is located on opposite axial sides from the transducer for vibrating the cone at a resonant frequency of less than about 200 kHz. In conventional nozzle type ultrasonic atomizers, the liquid is fed into an atomizing nozzle and then flows through or over a piezoelectric transducer and horn, which vibrate at ultrasonic frequencies to produce short wavelengths that atomize the liquid. U.S. Pat. No. 5,152,457 discloses that conventional ultrasonic atomizing nozzles incorporate a low-frequency electrical input from 25 to 120 kHz, two piezoelectric transducers, and a horn to produce weight mean droplet diameters in the range of 25 to 100 microns. Other conventional ultrasonic atomizers of the nebulizer type have been used in medical applications to produce droplets in the range of 1 to 5 microns.
It has been found that operation of transducers at about 200 kHz or above for prior art nozzle type atomizers causes so much heating that system failure will result with prolonged use. Without the resonant air assistance of U.S. Pat. No. 5,687,905, extremely small drops can""t be made with the nozzle type atomizers due to low (less than 200 kHz) frequency that may be applied to those nozzles. The nozzle lengths must equal an effective half wavelength so that maximum amplitude is obtained at the tip. A nozzle type device that has been long available is the ultrasonic nozzle (Sono-Tek Model 8700-120, Milton, N.Y.) with a central channel (0.93xc2x10.02 mm diameter) for liquid flow. The Sono-Tek ultrasonic nozzle consists of a pair of washer-shaped ceramic (PZT) piezoelectric transducers and a titanium resonator. The transducers, surrounding the central channel, are sandwiched in the titanium resonator located in the large diameter (about 3.6 cm) portion of the nozzle body. The piezoelectric transducers receive an electrical input at the nozzle resonant frequency from a broadband ultrasonic generator (Sono-Tek Model 06-05108), and convert the input electrical energy into mechanical energy of vibration. The nozzle is a half wavelength design with a resonant frequency (f) of 120 kHz. It is geometrically configured such that excitation of the piezoelectric transducers creates a standing wave through the nozzle, with the maximum vibration amplitude occurring at the nozzle tip. The outside diameter of the nozzle tip and the length of the front horn measure 3.12 mm and 1.4 cm, respectively. As a liquid jet issues from the nozzle tip, a liquid capillary wave is initiated by the ultrasound. The capillary wave travels axially along the jet in the direction of the liquid flow, and its amplitude grows exponentially due to amplification by the air blowing around it. Atomization occurs when the amplitude becomes too great to maintain wave stability. It is known to make a single horn stage nozzle having a single cone and an overall length in multiple half wavelengths, although it is also well known that the nozzle tip surface area is dramatically reduced by such multiplication of half wavelengths in the overall length.
The other capillary wave atomizer is the nebulizer type. U.S. Pat. No. 4,271,100 shows such a type. Its transducer operates at 200 kHz to 10 MHz. However, liquid has to be delivered to a vibrating surface. The advantage of the nebulizer type is clear. The higher operating frequency produces much smaller drops than those possible from the nozzle type (without the air assistance of U.S. Pat. No. 5,687,905). However, the nebulizer type requires a large energy input to generate its smaller drops and to improve surface area for atomization many strange configurations have been proposed in the prior art, such as those of U.S. Pat. Nos. 4,978,067, 4,726,522 and 4,350,302.
The capillary wave mechanism of ultrasonic atomization of a liquid jet in the nozzle type has been well accepted since its first demonstration in about 1962. Specifically, capillary waves are formed in the liquid film of a pressurized, flowing liquid stream contacting a solid surface that is vibrating at frequencies from 10 kHz to less than 200 kHz. An increase in the vibrational amplitude of a vibrating surface results in a proportional increase in the amplitude of the liquid capillary waves in the liquid film. An adequately designed ultrasonic atomizer will maintain contact between the vibrating solid surface and the flowing liquid stream until a wave amplitude is developed in the liquid film contacting the solid surface sufficient to cause atomization at some point after the liquid is no longer in contact with the vibrating surface. The vibrating solid surface is the inside of a tube through which the pressurized, flowing liquid stream moves, wherein the tube vibrates substantially parallel to the flow of the liquid stream.
Atomization in ultrasonic atomizers occurs when (1) the vibration amplitude of the solid surface increases the amplitude of the capillary waves of the liquid stream film above a level at which wave stability cannot be maintained and (2) the pressurized, flowing liquid stream is expanded into a lower pressure gas, as the continuous phase, of sufficient volume and/or flow rate to permit desired droplet formation. The resulting median drop size from ultrasonic atomizers is proportional to the wavelength of the capillary waves which is, in turn, determined by the ultrasonic frequency in accordance with the Kelvin equation.
There is a complete absence in the prior art of nozzle type atomization at and above 200 kHz due to mechanical and heating constraints. The present invention overcomes that limitation.
The present invention comprises a nozzle type atomizer with two or more aligned xe2x80x9chornxe2x80x9d stages as its ultrasonic resonator. The definition of a xe2x80x9chornxe2x80x9d stage is well known in the prior art as an effectively half wavelength length and a tapering shape cone with a central conduit. The present invention uses two to five, or more, horn stages integrally attached end to end. The dramatic improvement in amplitude of vibration at the tip of the nozzle is without precedence in the prior art. The present invention makes application of transducer vibration at greater than 200 kHz possible. The present invention reduces the required applied energy for generating the necessary capillary wave at the tip by the discovery of amplitude multiplication with two or more horn stages. The more specific example below and the drawing figures show this unexpected amplification in more detail.
The generally tapering shape of a horn stage means that cross section mass is reduced toward its distal end. At the start of the next integrally attached horn stage, the cross section mass is relatively suddenly increased to an effective inertial mass. The shape of the cross sections of the horn stages has been made effective by the present invention in both conical and substantially rectangular cross sections.
The preferred material of the horn stages is silicon or glass and similar composites and compounds of silicon effective for the objects of the present invention.
The transducer location in the invention may be at a flat side of a base section, embedded within the base section or affixed to a base of the base section. Thus, the invention can have one to four transducers in separated locations. The transducers include those with a single layer as well as the types that are electrically parallel and mechanically serial layout. The invention embodiments with more than one separated transducer are adapted to have effective electrical connection to a power supply where all transducers operate at an identical resonant frequency in coordination to produce capillary wave atomization.
The present invention is also a method of obtaining drops in the size typical of the operation of nebulizer types although from a nozzle type device.
The present invention is a method for nozzle type devices to operate at and above about 200 kHz.